I have my Grandfather's Ithaca SxS (I think its the Flues model) that I know he bought new, and to his order in 1909. 26" choked full/full, and stock made to his dimensions.
The gun has "fluid steel barrels" and is safe for smokeless powder. 3 or 3 1/4dr eq loads are fine in these guns (provided the guns are mechanically sound).
I have a letter from Ithaca to my Grandfather, dated 1949, he was asking them about the guarantee on the springs (never to set), and they replied it was still good. The letter included a short message from the VP at the time, saying to avoid the "express" loads, as they are not needed and "akin to threading a needle with a bulldozer" (his exact words,
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Your gun would probably stand a limited amount of shooting with 3 3/4dr eq loads, but I would NOT recommend it.
Knew one fellow who had a similar era 20ga, shot Express and about the 3rd (or 4th) case of shells he went through the receiver cracked.
To be sure, shoot only 3 or 3 1/4 dram equivalent shells. Factory shells in this range are almost always #6 or smaller shot, so handloading might be in order if you want something else.
Grandpa loaded his own, and was rather fond of soft lead #1s, and "punkin balls".
he wasn't much of a bird hunter, just a farmer who "killed a fox at 40 rod"....